Tiny bubbles within the glass can also be a sign of a handmade bottle.
The strength of the bottle wall may vary from top to bottom, visible by small variations of transparency of the colored glass.
After the method of pressing had been perfected, bottles where made by pressing a piece of molten glass into an iron mold with the help of a plunger.
This still left the problem of the bottom and neck, but very soon that was solved by a combination of the pressing method with the blowing method.
Whether this is also found in pre-industrial bottles I can not tell, since I have never seen any seals on this type of bottle.